I had the same problem yesterday after doing a maintenence cleaning.
Everything worked fine until I did the cleaning then the yellow did not work. This happens more when you are low on ink. Rather than do a powerful cleaning and wasting a lot of ink plus wearing out the heads I did this.
Plug the drains for the heads that are clogged or use a crimp. Since you need to hook up a syringe it is better to plug it.
http://www.jetsignage.com/roland/cleaning1.JPG
Fill up caps that are clogged with cleaning fluid and push heads back onto cap to soak.
Then
hook up a syringe to the drain tube and pinch the supply tube on the side of the head that is working and pull on the syringe at the same time. This will pull ink into the head to the side that was not working. You only need to pull a little bit of ink. It should have resistance. If not your cap seals may be damaged. If you do not pinch the supply tube it may just pull to the good side and not fix the problem.
Reconnect the drain tube and do a test print to see if it worked. I also print a 4 color row of ink to get the ink flowing.